Here is the user manual for the BH-MSD-2A Micro stepping driver.
Page 5 shows the two tables giving the connections:
You have already connected P2, and that leaves P1 remaining.
The timing diagram below shows the state of the,
ENA signals required
ENA input can either be held high, by connecting to VCC or connected to a digital output pin of the Arduino, to provide basic ON/OFF control.
This thread, arduino uno with stepper motor, the OP is using a very similar controller to yours, except that theirs is the 6 A version (BH-MSD-6A). Their code, in the thread has a lot of unnecessary code, and apparently has a number of issues. A stripped down version, of the same code, is as follows:
int stepper_Enable=5; //STEPPER MOTOR ENABLE PIN : PUL-
int stepper_Pulse=6; // STEPPER MOTOR PULSE PIN : PUL-
Timer1.attachInterrupt( timerIsr ); // attach the service routine here
pinMode(stepper_Pulse,OUTPUT); // Analog pin A2 is used for ENABLING the steeper motor
pinMode(stepper_Enable,OUTPUT); // Analog pin A2 is used for DIRECTION the steeper motor
Timer1.initialize(dutycycle_time); // set a timer of length 100000 microseconds (or 0.1 sec - or 10Hz => the led will blink 5 times, 5 cycles of on-and-off, per second)
if(analogRead(A3)==LOW ) // STOPPING THE STEPPER MOTOR
if(analogRead(A0)==LOW && rpm<300 ) ///increasing speed of stepper motor
if(analogRead(A1)==LOW && rpm>0) ///decreasing speed of stepper motor
if(analogRead(A2)==LOW ) //STARTING THE STEPPER MOTOR
// Toggle LED
digitalWrite( stepper_Pulse, !digitalRead( stepper_Pulse ));
Note that while this should get you started, as the OP states in the thread there are issues with it, that will need fixing.
(I have to go now to fix some Marlin firmware, but I'll come back to this answer later)